[19.11.x] Bug 25858: Don't attempt to set flag if already set

Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>

installer/data/mysql/updatedatabase.pl

@@ -20364,7 +20364,7 @@ if( CheckVersion( $DBversion ) ) {
     });
 
     $dbh->do(q{
-        UPDATE borrowers SET flags = flags + (1<<12) WHERE flags & (1 << 11)
+        UPDATE borrowers SET flags = flags + (1<<12) WHERE flags & (1 << 11) AND !(flags & (1 << 12))
     });
 
     SetVersion( $DBversion );